Fruit-damaging fly could hit record population in Northwest this year

Apr 23, 2013

Small and stone fruits are preferred foods of the spotted wing drosophila, an invasive pest that first arrived in the U.S. in 2008. Credit: Vaughn Walton

The spotted wing drosophila fly, which lays its eggs in fruit and makes it unmarketable, could reach record population levels in the Pacific Northwest this year, according to Oregon State University researchers.

"All indications estimate this season will be similar or worse than 2012, which was the worst on record," said Vaughn Walton, an entomologist with the OSU Extension Service. "Winter and spring temperatures in the Pacific Northwest have been warmer than last year, and heat equals larger populations of spotted wing drosophila."

Originally from Asia, the spotted wing drosophila was first found stateside in California in 2008 and has since spread across the continent. The insect lays its eggs in ripe and ripening small and stone fruits, and its developing larvae eat the fruit. The cosmetic imperfections caused by the larvae make the fruit undesirable to most consumers.

The fly's favorite fruits include blueberries, cherries, blackberries, raspberries, peaches and plums. The pest has not impacted wine grapes so far, Walton added.

Walton expects spotted wing drosophila populations in the Pacific Northwest to rapidly build through July and August when most susceptible fruits ripen.

The economic stakes are high. In Oregon alone, farmers grew $198 million of berries in 2012, with blueberries accounting for $108 million of that, according to a report by the U.S. Department of Agriculture. Growers also sold $74 million of sweet cherries that year, the report said.

In the absence of detection and control measures, Oregon's small and stone fruit industry could lose $31 million per year, according to a report by the Giannini Foundation of Agricultural Economics at the University of California.

Since the discovery of the pest in Oregon, OSU has been collaborating with scientists in California and Washington to better understand it and help growers deal with it. For example, researchers at OSU are seeing if a parasitic wasp that is native to the United States, known as Pachycrepoides vindemmiae, can be used to control the spotted wing drosophila. It lays its eggs in the fly's pupae, thus killing them.

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OSU will also lead a trip to South Korea in August to search for and collect other similar wasps, including one known as Asobara japonica that lays its eggs in the spotted wing drosophila's larvae. Over the next few years, researchers will study these wasps in quarantine to determine if it attacks only the fly's larvae. If tests show the wasp does not harm other insects, Asobara japonica and others could be released in the U.S. in three to five years.

For now, OSU has found that insecticides are the best way to control the pest. OSU pesticide evaluator Joe DeFrancesco tested various compounds for use on strawberries, blueberries and caneberries to see which are most effective. OSU entomologist Peter Shearer has conducted similar work on cherries. The top-performing pesticides are on OSU's website at bit.ly/SWD_GrowerInfoOSU.

"To protect against severe economic damage, we've seen farmers spraying more than usual – and this year will probably be no exception," said Shearer. "If farmers use proper sprays at proper times, they should be able to prevent the flies from damaging fruit."

Last year, farmers in the Willamette Valley and Oregon's Mid-Columbia Basin sprayed an average of five to nine times to control spotted wing drosophila at an average cost of $169 an acre, said Walton. Before the fly landed in Oregon, the state's small fruit growers typically sprayed only twice a year to manage other pests, Shearer said. Oregon's blueberry growers alone spent $6 million last year to manage the spotted wing drosophila, Walton estimates.

OSU is also investigating the impact of cold weather on the insect's survival. Early data suggest that some adults can survive fluctuating conditions and can live for 150 days in the winter. Low humidity appears to negatively impact the fly's survival and reproduction, but tests are still ongoing to confirm these findings.

Additionally, OSU researchers have also helped develop an interactive map that estimates the fly's population throughout the U.S. based on temperature and weather conditions. In the mid-Willamette Valley, data suggest that three to five generations of the pest emerge during each growing season.

OSU is also advising growers to monitor for the fly by hanging homemade traps containing apple cider vinegar in plastic cups punctured with small holes that lure in the insect. Researchers are working to develop better baits and traps that catch the spotted wing drosophila earlier in the ripening season to help growers determine when to treat for the pest.

In addition, Bernadine Strik, a berry crops specialist with the OSU Extension Service, is monitoring the presence of the pest in an organic research plot and using organically-approved methods to control the fly.

More information on the fly is on OSU's website at www.spottedwing.org. The site features guides to identify the fly, advice for gardeners and commercial growers, and updates on OSU's research. It also contains links to the following guides published by the OSU Extension Service:

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